1. Field of the Invention
This invention deals with the problems associated with very accurate control of the temperature of a food case or other refrigerated environment. In certain applications it is desirable to maintain extremely close tolerances on the refrigerated temperature in order to allow the temperature reading to be lowered as much as possible and yet still be sure that a certain absolute minimum temperature will be observed.
For example, if the items stored in a refrigerated environment contain water as a constituent and should never be allowed to freeze, then the absolute minimum temperature should be predetermined at approximately 33.degree. F. On the other hand, it is also desirable to maintain the temperature as close as possible to 33.degree. without going lower. Therefore, it becomes necessary to maintain the desired temperature within very close tolerances.
In refrigerated food cases which must be maintained at above 32.degree. F, the above conditions are the exact requirements. It is desirable to cool the case as low as possible without the fear of having one of the many variables cause the freezing of the food product. Many such variables exist such as ambient air temperature, humidity changes, variable lighting loads at day and night, varying customer use, etc. To quickly adapt to such rapid changes in conditions and maintain the temperature within the desired close tolerance limits requires a very rapdily responsive temperature control system.
2. Description of the Prior Art
One element of the present concept makes use of a well-known technological device commonly referred to as a "heat pipe." (The "heat pipe" phenomenon is fully explained in the November, 1968 issue of Mechanical engineering) The heat pipe has been used in a wide variety of technologies for the transportation of heat, expecially when the amounts of heat to be transported are very large or when there are confining spacial limitations on the area available for the heat exchange device. Heat pipes have been shown to have much greater heat exchange capacities than the best of metals.
Basically the heat pipe is a closed evacuated chamber with a volatile fluid therein having the desired temperature-pressure relationship at the particular temperature at which it will be operating. The basic theory of operation is that as one end of the chamber is warmed some liquid is thereby vaporized causing the vapor pressure in the proximate region to increase; a slight temporary pressure gradient is created across the gaseous atmosphere in the chamber. Thus, vapor flow is initiated with the warmer vapor flowing to the lower vapor pressure area in the cooler section of the chamber. When the warm vapor reaches the cool zone, it distributes its heat to the surrounding vapor and condenses. This freshly condensed liquid then flows back to the previously warmed section either by gravitational flow or, alternatively, by capillary flow through a wick extending from one end of the chamber to the other end. This vapor flow occurs at a very rapid pace such that in the average heat pipe, the temperature difference from one end to the other is very slight. While most past uses of the heat pipe have been directed to the very favorable characteristic of large heat flow volume, the present invention makes use of the averaging and communicating aspects of the phenomenon which has been largely ignored heretofore in the field of refrigerated food cases. The heat pipe has an inherant ability to average the temperature which it senses along its length and to communicate this average temperature to a remote location. Both of these capabilities are extremely useful in the rapidly responsive control needed for monitoring refrigerated environments.
Many manners of refrigeration control have been used to attempt to closely control the temperature tolerances but none have been able to overcome the variables introduced by the wide variations in operating conditions to which refrigerated food cases are exposed. One of the foremost attempts has been a two-stage system which, firstly, places a thermistor device in the primary air discharge area of a standard refrigerated case. In this manner, as the environment temperature increases or decreases the information is electrically conveyed to the refrigerating apparatus. There are several difficulties with this system since external air currents or ambient air humidity could generate a false reading and cause excess or insufficient refrigeration. A problem inherant with this system is that only one small section, the discharge section of the refrigerated environment is being sensed and, as such, small localized variations are not being considered.
The second stage of this system is the off-hours or night control. At night when the store is closed and the case is not being opened and the external air currents are minimal, the great decrease in refrigeration requirements can not be taken into account. The single largest factor is the turning off of the case lights at night which, in and of itself alone, requires a case temperature upward shift of as much as 3.degree. F to prevent freezing. Thus a second overriding control is necessary to shift the thermostat up 3.degree. F during the night.
Even with thus dual means of control, it is apparent that a device for sensing direct product temperature or the direct area temperature of product storage would be desirable. Even though very close control of the discharge air temperature is possible with the two-stage system, the product temperature will change with variations in other variables which are not being effectively monitored such as ambient air temperature and humidity, case light heat, store area air current and customer usage, case service and rear door openings, and varying usage of night covers. These extraneous variables are now able to be sensed by the invention described herein.